Fire suppression equipment has been used for some time to suppress and control fires. In its most basic form, fire suppression equipment typically includes a spray nozzle attached to a supply of fire suppressant (water) at a relatively high pressure. Traditional sprinkler-based fire-suppression systems typically remain inactive and become operational only when the heat generated by a fire causes a low temperature solder within one of the sprinkler's nozzles to melt (“thermal-reactive” fire suppressant systems). As the solder becomes molten, the stopper that previously prevented the flow of fire suppressant is released and the fire suppressant is allowed to flow.
The National Fire Protection Association (NFPA) issues a standard known as NFPA-13 (also known as “Standard for the Installation of Sprinklers Systems”). NFPA-13 defines requirements for the types of sprinkler systems described above.
NFPA-13 recognizes three general hazard categories for sprinkler systems: light, ordinary and extra hazard. As defined by NFPA-13, light hazard occupancies are those situations where the quantity and combustibility is low and fires with relatively low rates of heat-release are expected. Ordinary hazard occupancies are those situations where the quantity and/or combustibility of the contents is equal to or greater than that of the light hazard, ranging from low to high, where the quantity of combustibles is moderate and stock piles so not exceed twelve feet, such that fires with moderate to high rates of heat release are expected. Extra hazard occupancies are those where quantity and combustibility of contents is very high and flammable or where combustible liquids, dust, lint or other materials are present, such that the probability of rapidly developing fires with high rates of heat release is very high.
Many sprinkler systems specified in NFPA-13 were designed to control a fire rather than to extinguish it. Such sprinkler systems are generally designed to limit the size of a developing fire and to prevent it from growing and spreading beyond the general area of origin. The concept of fire suppression was only started when the first Early Suppression Fast Response (ESFR) sprinklers were introduced in 1988. This fire suppression concept evolved by examining how the effects of sprinkler sensitivity and water distribution characteristics could be combined to achieve early fire suppression.
ESFR sprinklers achieve fire suppression by responding more quickly to fire hazard than standard sprinklers and provide adequate discharge to suppress the fire before a severe fire plume develops. The concept is that if a sufficient amount of fire suppressant can be discharged in the early phases of a fire and if the fire suppressant penetrates the developing fire plume, fire suppression can be achieved. Early suppression is determined by satisfying the following three factors: thermal sensitivity, required delivery density (RDD), and actual delivery density (ADD).
Response time index (RTI) is a measurement that is used to quantify the thermal sensitivity of a sprinkler system. RTI is a function of the thermal sensitivity of the operating element of the fire suppressant system, the temperature rating of the fire suppressant system, and the distance of the fire suppressant system relative to the fire hazard.
It is recognized in the art of fire suppression equipment that traditional thermal-reactive fire suppression equipment is subject to thermal lag. Thermal lag is associated with the mass of the traditional thermal-sensitive operating element to sense heat in gases from a fire. Thermal-sensitive sensors of traditional thermal-reactive fire protection systems rely on detecting the heat of gases from a fire that accumulate near the sensor. It is the sensing of the heat from such gases that is used by traditional thermal-reactive fire suppression systems to activate alarms and to activate the release of fire suppressants. In order to activate a traditional thermal-reactive fire suppression system, the temperature of the gases released from the fire that accumulate near the sprinkler head must reach a very high value before the sprinkler system will be activated.
Due to the thermal lag in a traditional thermal-reactive fire suppressant system, the response time of such equipment is long. Because the response time is long, a fire can develop into a significant fire hazard before the fire suppressant system is activated. On the other hand, a thermal-reactive fire suppressant system with extra-sensitive thermal-sensitive elements can be prematurely activated, such as by a strong fire plume causing the activation of a sprinkler far away from the fire hazard. If a fire suppressant system sprinkler is activated prematurely, such as a sprinkler that is far from the fire source and not directed to, or not configured to spray fire suppressant with sufficient force to reach, the fire source, such premature activation is not an aid in the suppression of the fire hazard. The need to quickly respond to the fire hazard and the need to prevent inadvertent activation of nearby nozzles render traditional methods of fire detection ineffective.
Once activated, a fire suppressant system needs to douse the fire with sufficient fire suppressant such that the actual delivery density (ADD) over the ensuing fire exceeds the required delivery density (RDD) to suppress the fire. The RDD depends on the strength of the ensuing fire and the combustibility of the materials stored in the vicinity requiring fire protection. ADD is a function of fire plume velocity, momentum and size of the water droplets, and the distance that water must travel from the sprinkler. Once activated, the nozzle from a traditional fire suppressant system spreads the water in a generally dispersive circular pattern and typically reaches the fire with only the aid of gravity. Due to the almost random nature of the type of a distribution system, the delivery of the water on the fire hazard is not very effective.
These three measurements—RTI, RDD, and ADD—are the controlling factors that define the time-dependent nature of early fire suppression. The earlier the water is applied to a growing fire, the lower the RDD will be and the higher the ADD will be. In other words, the faster the sprinkler response (the lower the RTI), the lower the RDD and the higher the ADD. Conversely, the later the water is applied (the higher the RTI), the higher the RDD and the lower the ADD.
When the ADD is less than the RDD, the sprinkler discharge is no longer effective enough to achieve early fire suppression. Thus, it is clear that early fire suppression depends on the ability of a fire suppression system to detect a fire hazard quickly and to react with the proper response to ensure that the sufficient fire suppressant necessary to suppress the fire is delivered.